The vitamin alphabet: All about A

Published: September, 2012

Vitamins are essential for a healthy body. Acting together, these compounds help shore up bones, heal wounds, support the immune system, convert food into energy, and repair cell and tissue damage — just for starters.

As tempting as it is to troll the vitamin aisle at the health food store, it's best to get the vitamins you need from foods, not from multivitamin pills or supplements. Extensive research shows that eating a healthy diet can cut your risk for chronic disease and disability. The evidence for taking vitamins is less convincing.

Vitamin A and carotenoids

Colorful fruits and vegetables are rich in pigments known as carotenoids. The body converts two carotenoids, beta carotene and alpha carotene, into vitamin A. Animal-based foods, including liver, milk, and eggs contain another form of vitamin A, called retinol. Most fat-free milk and dried nonfat milk solids are fortified with vitamin A to replace the amount lost when the fat is removed. It is also added to many breakfast cereals.

Vitamin A and your health

Research suggests that vitamin A plays many roles in health and disease. The examples below underscore that it's important not to overdo it with vitamin A or any vitamin.

Cancer. Studies suggest that people who eat foods rich in beta carotene and vitamin A are less likely to develop many types of cancer, especially lung cancer. But be aware: research also shows that long-term use of high-dose beta carotene supplements are linked with an increased risk of lung cancer in smokers and former smokers.

Fractures. Several observational studies show that too much retinol — through diet or supplements — may weaken bones and increase the likelihood of fractures. The risk appears when people consume more than double the recommended daily amount for preformed vitamin A and may be limited to those who don't get enough vitamin D.

Eye diseases. Vitamin A is essential to healthy eyes, as are lutein and zeaxanthin, two carotenoids that aren't transformed into vitamin A. Lutein and zeaxanthin are the only carotenoids found in the retina of the eye. Spinach and kale, two lutein-rich vegetables, appear to moderately reduce the risk of cataract (the clouding of the normally clear lens of the eye).

Vitamin A

Recommended Daily Allowance/Adequate Intake

Upper tolerable limit

Good food sources

Essential for vision

Keeps tissues and skin healthy

Plays an important role in bone growth

Men: 900 micrograms (mcg) (3,000 IU)

Women: 700 mcg (2,333 IU)

Some supplements report vitamin A in IU.

3,000 mcg (about 10,000 IU)

Sources of retinoids: beef, liver, eggs, shrimp, fish, fortified milk, Cheddar cheese, Swiss cheese

Sources of beta carotene: sweet potatoes, carrots, pumpkins, squash, spinach, mangoes, turnip greens

For more details on the benefits of Vitamin A and advice on making sure you get the proper amount of all the vitamins and minerals in your diet, buy Vitamins and Minerals by Harvard Medical School.

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